builtin/commit.c: switch to strbuf, instead of snprintf()

builtin/commit.c: switch to strbuf, instead of snprintf()

Switch to dynamic allocation with strbuf, so we can avoid dealing
with magic numbers in the code and reduce the cognitive burden from
the programmers.  The original code is correct, but programmers no
longer have to count bytes needed for static allocation to know that.

As a side effect of this change, we also reduce the snprintf()
calls, that may silently truncate results if the programmer is not
careful.

Helped-by: René Scharfe <l.s.r@web.de>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>